The present invention generally relates to improving the transmission of light through optical materials, such as spectacle lenses and, at the same time, reducing reflection of stray light that leads to glare from optical materials. The invention further relates to controlling the perceived color of light reflected from the surface of optical materials.
All uncoated, optically transparent materials reflect a portion of incident light. The amount of reflection varies with the wavelength, polarization, and angle of incidence of the light as well as the wavelength-dependent refractive index, n, of the material. This Fresnel reflection is described by Maxwell's equations for electromagnetic radiation, as known to those practiced in the art of optics and described, for example, by M. Born and E. Wolf in Principles of Optics, New York, Pergammon Press (1980). It is also known that layers of transmissive materials with refractive indices different from that of the substrate can reduce the amount of reflection. The amount of this reduction depends on the wavelength-dependent refractive index of the coating materials and their thickness as well as the wavelength, polarization, and angle of incidence of the light. The design and manufacture of these anti-reflection (AR) coatings is thoroughly described in Chapters 3 and 9 of H. A. McLeod, Thin Film Optical Filters, New York, McGraw-Hill (1989).
The sensitivity of the human visual system also varies with the wavelength of light and the angle with which it enters the eye, as described, for example, in Color Science: Concepts and Methods, Quantitative Data and Formulae by Gunter Wyszecki and W. S. Stiles (New York:Wiley) (1982) and Visual Perception by Nicholas Wade and Michael Swanston (London:Routledge) (1991). A problem therefore is to choose the coating thickness and composition so that the angular and wavelength variation of Fresnel reflection from the coated article as perceived by the human visual system is minimized.
Known AR coatings use one or more thin layers of inorganic oxides, nitrides, or fluorides to achieve a reduction in reflection. Common thin-film materials used for such AR coatings are described in chapter 9 and Appendix I of Mcleod and include oxides of Al, Sb, Be, Bi, Ce, Hf, La, Mg, Nd, Pr, Sc, Si, Ta, Ti, Th, Y, and Zr. Mcleod's tabulation also includes fluorides of Bi, Ca, Ce, Al, La, Na, Pb, Li, Mg, Nd, Na, and Th, as well as a few sulphides and selenides. A similar tabulation is found in table 4.1 on page 179 of Optics of Multilayer Systems (Sh. A. Furman and A. V. Tikhonravov, Editions Frontieres:Gif-sur Yvette Cedex- France, 1992). The number of layers and their compositions are generally chosen based on auxilliary constraints including hardness or scratch resistance, adhesion, durability, ease of deposition, cost, and other factors familiar to those practiced in the art of optical coatings. However the layer thicknesses are generally adjusted to minimize the proportion of incident light that is reflected (reflectance) at normal incidence and one or more specified wavelengths. A problem therefore is to choose a set of layer thicknesses that minimize or significantly reduce the amount of reflected light that can be perceived by the human visual system over all relevant angles and wavelengths.
As described above, the amount of reflectance from a coated article varies with angle and wavelength. When uncoated spectacle lenses are worn, a person looking at the wearer perceives a reflection of light from the environment, i.e., "glare." The color of this reflection is, for uncoated lenses, typically that of the ambient light source(s) because the variation of reflection with wavelength from an uncoated spectacle is quite small. This result is generally true for mildly dispersive optical materials such as glass, polycarbonate, polymethylmethacrylate, and other spectacle lens materials. A plot of the wavelength and angle dependent reflectance for glass is shown in FIG. 1.
The amount of reflected light from an AR coated article varies more dramatically with wavelength and angle, so that the perceived color of the reflection may differ from that of the light source. As this color influences the cosmetic quality of a spectacle lens, and other optical substrates, it is therefore, desirable to reduce reflection while controlling the perceived color of reflected light.